JavaScript - 无法设置未定义的属性
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JavaScript - cannot set property of undefined
提问by StackOverflowNewbie
My code:
我的代码：
var a = "1",
b = "hello",
c = { "100" : "some important data" },
d = {};
d[a]["greeting"] = b;
d[a]["data"] = c;
console.debug (d);
I get the following error:
我收到以下错误：
Uncaught TypeError: Cannot set property 'greeting' of undefined.
未捕获的类型错误：无法设置未定义的属性“问候语”。
I'm trying to do something similar to an associative array. Why isn't this working?
我正在尝试做一些类似于关联数组的事情。为什么这不起作用？
回答by zzzzBov
you never set d[a]to any value.
您从未设置d[a]为任何值。
Because of this, d[a]evaluates to undefined, and you can't set properties on undefined.
因此，d[a]计算结果为undefined，并且您不能在 上设置属性undefined。
If you add d[a] = {}right after d = {}things should work as expected.
如果您d[a] = {}在d = {}事情应该按预期工作之后立即添加。
Alternatively, you could use an object initializer:
或者，您可以使用对象初始值设定项：
d[a] = {
greetings: b,
data: c
};
Or you could set all the properties of din an anonymous function instance:
或者您可以设置d匿名函数实例中的所有属性：
d = new function () {
this[a] = {
greetings: b,
data: c
};
};
If you're in an environment that supports ES2015 features, you can use computed property names:
如果您处于支持 ES2015 功能的环境中，则可以使用计算属性名称：
d = {
[a]: {
greetings: b,
data: c
}
};
回答by vol7ron
You have to set d[a]to either an associative array, or an object:
您必须设置d[a]为关联数组或对象：
d[a] = [];d[a] = {};
d[a] = [];d[a] = {};
Without setting, this is what's happening:
没有设置，这就是正在发生的事情：
d[a] == undefined, so you're doing undefined['greeting']=b;and by definition, undefined has no properties. Thus, the error you received.
d[a] == undefined，所以你正在做undefined['greeting']=b;，根据定义， undefined 没有属性。因此，您收到了错误。
回答by Polaris878
The object stored at d[a]has not been set to anything. Thus, d[a]evaluates to undefined. You can't assign a property to undefined:). You need to assign an object or array to d[a]:
存储在的对象d[a]尚未设置为任何内容。因此，d[a]评估为undefined。您不能将属性分配给undefined:)。您需要将一个对象或数组分配给d[a]：
d[a] = [];
d[a]["greeting"] = b;
console.debug(d);
回答by wukong
In javascript almost everything is an object, nulland undefinedare exception.
在 javascript 中，几乎所有东西都是对象，null并且undefined是例外。
Instances of Arrayis an object. so you can set property of an array, for the same reason,you can't set property of a undefined, because its NOTan object
实例Array是一个对象。所以你可以设置数组的属性，出于同样的原因，你不能设置未定义的属性，因为它不是一个对象
回答by iohzrd
i'd just do a simple check to see if d[a] exists and if not initialize it...
我只是做一个简单的检查，看看 d[a] 是否存在，如果没有初始化它......
var a = "1",
b = "hello",
c = { "100" : "some important data" },
d = {};
if (d[a] === undefined) {
d[a] = {}
};
d[a]["greeting"] = b;
d[a]["data"] = c;
console.debug (d);
